Methods and devices for processing polymer films

ABSTRACT

Stretched polymeric films can be used in a variety of applications, including optical applications. The stretching conditions and shape of the stretching tracks in a stretching apparatus can determine or influence film properties. Take-away systems can be used to receive the film after stretching. The configuration of the take-away system can, in at least some instances, influence final film properties.

This application is a divisional application of U.S. Ser. No.10/306,593, filed Nov. 27, 2002, the disclosure of which is herebyincorporated by reference herein.

FIELD OF THE INVENTION

Generally, the present invention relates to methods and devices forstretching polymer films and the films obtained by the methods anddevices. The present invention also relates to methods and devices forstretching polymer films using take-away systems to receive the polymerfilm after stretching.

BACKGROUND OF THE INVENTION

There are a variety of reasons to stretch polymer films. Stretching canenhance or generate desired mechanical, optical, and other filmproperties. For example, polymer films can be stretched to provide adesired degree of uniaxial or near uniaxial orientation in opticalproperties. In general, perfect uniaxial orientation of a birefringentpolymer results in a film (or layers of a film) in which the index ofrefraction in two of three orthogonal directions is the same (forexample, the width (W) and thickness (T) direction of a film, asillustrated in FIG. 4). The index of refraction in the third direction(for example, along the length (L) direction of the film) is differentfrom the indices of refraction in the other two directions. Typically,perfect uniaxial orientation is not required and some degree ofdeviation from the optimal conditions can be allowed depending on avariety of factors including the end-use application of the polymerfilm.

In optical applications, a uniaxially oriented film can provide usefuloptical properties such as more uniform performance across a variety ofdifferent viewing angles. Other applications can also benefit fromuniaxial or near uniaxial orientation of a polymer film. For example,uniaxially oriented films are more easily fibrillated or torn along theorientation direction.

SUMMARY OF THE INVENTION

Generally, the present invention relates to methods and devices forprocessing polymer films. One exemplary embodiment is an apparatus forprocessing a film. The apparatus includes a conveyor and an isolatedtake-away system. The conveyor is configured and arranged to convey thefilm along a machine direction within the apparatus. The conveyorincludes gripping members that are configured and arranged to holdopposing edge portions of the film. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system includes opposing tracks and gripping membersconfigured and arranged to grasp opposing take-away regions of the filmafter a desired amount of stretching and convey the opposing take-awayregions of the film along the opposing tracks. The opposing tracksdefine a region in which at least a portion of at least one of theopposing tracks is angled with respect to the machine direction. In someexemplary implementations, the opposing tracks may be disposed at anangle of at least 1° with respect to the machine direction.Alternatively or additionally, in some exemplary implementations, thetracks may be angled with respect to each other, for example, away fromor toward each other.

Yet another exemplary embodiment is an apparatus for processing a film.The apparatus includes a conveyor and an isolated take-away system. Theconveyor has gripping members that hold opposing edge portions of thefilm and convey, under influence of a drive member, the film along amachine direction within apparatus. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system includes a first set of opposing tracks, a second setof opposing tracks, a plurality of first gripping members configured andarranged to grasp opposing first take-away regions of the film after adesired amount of stretching and convey the film along the first setopposing tracks, and a plurality of second gripping members configuredand arranged to grasp opposing second take-away regions of the film andconvey the film along the second set of opposing tracks. The secondtake-away regions are disposed nearer a center of the film than thefirst take-away regions.

Another exemplary embodiment is an apparatus for processing a film. Theapparatus includes a conveyor and an isolated take-away system. Theconveyor has gripping members that hold opposing edge portions of thefilm and convey, under influence of a drive member, the film along amachine direction within the apparatus. A portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film. The isolated take-away systemreceives the film from the conveyor after stretching the film. Thetake-away system has opposing tracks and a plurality of gripping membersconfigured and arranged to grasp opposing take-away regions of the filmafter a desired amount of stretching and convey the opposing take-awayregions of the film along the opposing tracks. The apparatus isconfigured and arranged to allow selection of a final transversedirection draw ratio of the film by changing a position of the isolatedtake-away system with respect to a position of the conveyor.

Another exemplary embodiment is an apparatus for processing a film. Theapparatus includes a conveyor, a stretching region, and apost-conditioning region. The conveyor is configured and arranged toconvey the film along a machine direction. The conveyor has grippingmembers that are configured and arranged to hold opposing edge portionsof the film. In the stretching region the gripping members areconfigured arranged to travel along diverging paths to stretch the film.The post-conditioning region is disposed after the stretching region andincludes at least one zone in which the gripping members are configuredand arranged to travel along converging paths.

The above summary of the present invention is not intended to describeeach disclosed embodiment or every implementation of the presentinvention. The Figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic top view of a prior art tenter apparatus used tostretch film;

FIG. 2 is a perspective view of a portion of film in the prior artprocess depicted in FIG. 1 both before and after the stretching process;

FIG. 3 is a block diagram showing steps according to one aspect of thepresent invention;

FIG. 4 is a perspective view of a portion of film in a uniaxialstretching process both before and after the stretching process;

FIG. 5 is a schematic illustration of one embodiment of the stretchingprocess and one embodiment of a stretching apparatus according to thepresent invention;

FIG. 6 is a schematic top view of a portion of a stretching apparatusaccording to the present invention;

FIG. 7 is an end view of the apparatus of FIG. 6;

FIG. 8 is a schematic illustration of a portion of the tracks of astretching apparatus illustrating one embodiment of a pre-conditioningregion of the stretching apparatus according to the invention;

FIG. 9 is a schematic illustration of one embodiment of adjustabletracks for a primary stretching region of a stretching apparatusaccording to the invention;

FIG. 10 is a schematic illustration of one embodiment of a take-awaysystem for a stretching apparatus according to the invention;

FIG. 11 is a schematic illustration of another embodiment of a take-awaysystem for a stretching apparatus according to the invention;

FIG. 12 is a schematic illustration of a third embodiment of a take-awaysystem for a stretching apparatus according to the invention;

FIG. 13 is a schematic illustration of a fourth embodiment of atake-away system for a stretching apparatus according to the invention;

FIG. 14 is a schematic illustration of a fifth embodiment of a take-awaysystem for a stretching apparatus according to the invention;

FIG. 15 is a schematic illustration of another embodiment of tracks fora primary stretching region of a stretching apparatus according to theinvention;

FIG. 16 is a schematic side cross-sectional view of one embodiment oftracks and a track shape control unit for a stretching apparatusaccording to the invention;

FIG. 17 is a schematic illustration of one embodiment of a take-awaysystem, according to the invention, for using in, for example, aconventional stretching apparatus such as that illustrated in FIG. 1;

FIG. 18 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus according to theinvention;

FIG. 19 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus according to theinvention illustrating the use of different stretching regions withdifferent parabolic configurations;

FIG. 20 is a graph of examples of suitable boundary trajectories for aprimary stretching region of a stretching apparatus according to theinvention including boundary trajectories that are linear approximationsto suitable parabolic or substantially parabolic boundary trajectories;

FIG. 21 is a schematic view of a portion of the track and track shapecontrol unit of one embodiment of FIG. 16; and

FIG. 22 is a schematic view of another portion of the track and trackshape control unit of one embodiment of FIG. 16.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is believed to be applicable to methods anddevices for stretching polymer films and the films made using themethods and devices. In addition, the present invention is directed tomethods and devices for stretching polymer films using take-away systemsto receive the polymer film after stretching. The polymer films can bestretched using these methods and devices to achieve uniaxial or nearuniaxial orientation, if desired. The methods and devices can also beused to achieve other orientation conditions.

The present invention is applicable generally to a number of differentpolymer films, materials, and processes. The present invention isbelieved to be particularly suited to the fabrication of polymer opticalfilms. The methods and devices can be used, if desired, to make opticalfilms or other films having one or more properties selected fromimproved optical performance, improved optical properties, increasedpropensity to fracture or tear in a controlled manner or direction,enhanced dimensional stability, better processability, easiermanufacturability, and lower cost when compared to optical films madeusing conventional methods and devices.

A variety of optical films can be stretched or drawn according to thepresent invention. The films can be single or multi-layer films.Suitable films are disclosed, for example, in U.S. Pat. Nos. 5,699,188;5,825,543; 5,882,574; 5,965,247; and 6,096,375; and PCT PatentApplications Publication Nos. WO 95/17303; WO 96/19347; WO 99/36812; andWO 99/36248 (the entire contents of each of which are hereinincorporated by reference). The devices and methods described hereininclude improvements, additions, or alterations to the devices andmethods described in U.S. patent application Ser. Nos. 10/156,347 and10/156,348 and U.S. Provisional Patent Application Ser. No. 60/294,490,all of which are incorporated herein by reference.

Films made in accordance with the present invention may be useful for awide variety of products including, for example, polarizers, reflectivepolarizers, dichroic polarizers, aligned reflective/dichroic polarizers,absorbing polarizers, and retarders (including z-axis retarders). Thepolymer films can be monolithic or multilayer polymer films. Thepolymeric films may also comprise layers of immiscible blends that formoptical effects such as diffusers or diffuse reflective polarizers, suchas described in U.S. Pat. Nos. 5,783,120; 5,825,543; 5,867,316;6,057,961; 6,111,696; and 6,179,948 and U.S. patent application Ser.Nos. 09/871,130 and 09/686,460, all of which are incorporated herein byreference. These polymer films can include coatings or additional layersthat are provided before or after drawing. Examples of some suitablecoatings and layers are described in U.S. Pat. No. 6,368,699,incorporated herein by reference. In some embodiments, the polymer filmsinclude additional polarizing elements such as melt extrudable orientingdyes, wire grid polarizing elements, and the like. One example of auseful construction is a film with a layer of polyvinyl alcohol (PVA)that is formed on the film, e.g. coated on the film prior to or afterstretching the film. The PVA can be post-processed to form a dichroicpolarizing layer, e.g. through an iodine staining, acid dehydration ordye embedding methods. The substrate may itself be a monolithic film ora multilayer construction with or without optical reflective power.Examples of PVA films suitable for use in this construction can be foundin U.S. Pat. No. 6,113,811, which is incorporated herein by reference.

One application of the particular films of the invention is as acomponent in devices such as, for example, polarizing beamsplitters forfront and rear projection systems or as a brightness enhancement filmused in a display (for example, a liquid crystal display) ormicrodisplay. It should also be noted that the stretcher described belowin accordance with the present invention may be used with a lengthorienter to make a mirror.

In general, the process includes stretching a film that can be describedwith reference to three mutually orthogonal axes corresponding to themachine direction (MD), the transverse direction (TD), and the normaldirection (ND). These axes correspond to the width, length, andthickness of the film, as illustrated in FIG. 4. The stretching processstretches a region 20 of the film from an initial configuration 24 to afinal configuration 26. The machine direction is the general directionalong which the film travels through a stretching device, for example,the apparatus as illustrated in FIG. 5. The transverse direction is thesecond axis within the plane of the film and is orthogonal to themachine direction. The normal direction is orthogonal to both MD and TDand corresponds generally to the thickness dimension of the polymerfilm.

FIG. 3 is a block diagram of a process according to the presentinvention. In step 30, the film is supplied or provided to an apparatusfor stretching the film. The process optionally includes apreconditioning step 32. The film is stretched in step 34. The film isoptionally post-conditioned in step 36. The film is removed from thestretching apparatus in step 38.

FIG. 5 illustrates one embodiment of a stretching apparatus and methodof the invention. It will be recognized that the process illustrated byFIG. 3 can be accomplished using one or more additional apparatuses,apart from a stretching apparatus (which at a minimum performs step 34of FIG. 3). These one or more additional apparatuses perform one or moreof the non-stretching functions (for example, functions represented bysteps 30, 32, 36 and 38) illustrated in FIG. 3 and shown in FIG. 5 asbeing performed by a stretching apparatus.

In the illustrated embodiment of FIG. 5, the apparatus includes a region30 where the film 40 is introduced into the stretching apparatus. Thefilm can be provided by any desirable method. For example, the film canbe produced in a roll or other form and then provided to stretchingapparatus. As another example, the stretching apparatus can beconfigured to receive the film from an extruder (if, for example, thefilm is generated by extrusion and ready for stretching after extrusion)or a coater (if, for example, the film is generated by coating or isready for stretching after receiving one or more coated layers) or alaminator (if, for example the film is generated by lamination or isready for stretching after receiving one or more laminated layers).

Generally, the film 40 is presented in region 30 to one or more grippingmembers that are configured and arranged to hold opposing edges of thefilm and convey the film along opposing tracks 64 defining predeterminedpaths. The gripping members 70 (see FIG. 7) typically hold the film ator near the edges of the film. The portions of the film held by thegripping members are often unsuitable for use after stretching so theposition of the gripping members is typically selected to providesufficient grip on the film to permit stretching while controlling theamount of waste material generated by the process.

One example of suitable gripping members includes a series of clips thatsequentially grip the film between opposing surfaces and then travelaround a track. The gripping members can nest or ride in a groove or achannel along the track. Another example is a belt system that holds thefilm between opposing belts or treads, or a series of belts or treads,and directs the film along the track. Belts and treads can, if desired,provide a flexible and continuous, or semi-continuous, film conveyancemechanism. A variety of opposing, multiple belt methods are described,for example, in U.S. Pat. No. 5,517,737 or in European PatentApplication Publication No. 0236171 A1 (the entire contents of each ofwhich are herein incorporated by reference). The tension of belts isoptionally adjustable to obtain a desired level of gripping.

A belt or clip can be made of any material. For example, a belt can be acomposite construction. One example of a suitable belt includes an innerlayer made of metal, such as steel, to support high tension and an outerlayer of elastomer to provide good gripping. Other belts can be used. Insome embodiments, the belt includes discontinuous tread to provide goodgripping.

Other methods of gripping and conveying the film through a stretcher areknown and may be used. In some embodiments, different portions of thestretching apparatus can use different types of gripping members.

Gripping members, such as clips, can be directed along the track by, forexample, rollers 62 rotating a chain along the track with the grippingmembers coupled to the chain. The rollers are connected to a drivermechanism that controls the speed and direction of the film as it isconveyed through the stretching apparatus. Rollers can also be used torotate and control the speed of belt-type gripping members. The beltsand rollers optionally include interlocking teeth to reduce or preventslippage between the belt and roller.

FIGS. 6 and 7 illustrate one embodiment of the gripping members andtrack. The gripping members 70 of this embodiment are a series of tenterclips. These clips can afford overall flexibility via segmentation. Thediscrete clips are typically closely packed and attached to a flexiblestructure such as a chain. The flexible structure rides along or inchannels along the track 64. Strategically placed cams and cam surfacesopen and close the tenter clips at desired points. The clip and chainassembly optionally ride on wheels or bearings or the like. As oneexample, the gripping members are tenter clips mounted on top and bottombearings rolling between two pairs of inner and outer rails. These railsform, at least in part, the track.

The edges of the gripping members define a boundary edge for the portionof the film that will be stretched. The motion of the gripping membersalong the tracks provides a boundary trajectory that is, at least inpart, responsible for the motion and drawing of the film. Other effects(e.g., downweb tension and take-up devices) may account for otherportions of the motion and drawing. The boundary trajectory is typicallymore easily identified from the track or rail along which the grippingmembers travel. For example, the effective edge of the center of thegripping member, e.g. a tenter clip, can be aligned to trace the samepath as a surface of the track or rail. This surface then coincides withthe boundary trajectory. In practice, the effective edge of the grippingmembers can be somewhat obscured by slight film slippage from or flowout from under the gripping members, but these deviations can be madesmall.

In addition, for gripping members such as tenter clips the length of theedge face can influence the actual boundary trajectory. Smaller clipswill in general provide better approximations to the boundarytrajectories and smaller stretching fluctuations. In at least someembodiments, the length of a clip face edge is no more that one-half,and can be no more than one-quarter, the total initial distance betweenthe opposing boundary trajectories or tracks.

The two opposing tracks are optionally disposed on two separate orseparable platforms or are otherwise configured to allow the distancebetween the opposing tracks to be adjustable. This can be particularlyuseful if different sizes of film are to be stretched by the apparatusor if there is a desire to vary the stretching configuration in theprimary stretching region, as discussed below. Separation or variationbetween the opposing tracks can be performed manually, mechanically (forexample, using a computer or other device to control a driver that canalter the separation distance between the tracks), or both.

Since the film is held by two sets of opposing gripping members mountedon opposing tracks, there are two opposing boundary trajectories. In atleast some embodiments, these trajectories are mirror images about an MDcenterline of the drawing film. In other embodiments, the opposingtracks are not mirror images. Such a non-mirror image arrangement can beuseful in providing a variation (for example, a gradient or rotation ofprincipal axes) in one or more optical or physical properties across thefilm.

Returning to FIG. 5, the apparatus optionally includes a preconditioningregion 32 that typically is enclosed by an oven 54 or other apparatus orarrangement to heat the film in preparation for stretching. Thepreconditioning region can include a preheating zone 42, a heat soakzone 44, or both. In at least some embodiments, there may be a smallamount of film stretching that occurs in order to set the contactbetween the gripping members and the film, as illustrated by theboundary trajectory of FIG. 8. In at least some instances, there may notactually be any stretching but the increase in separation between theopposing tracks may account, at least in part, for thermal expansion ofthe film as the film is heated.

FIG. 8 illustrates a supply region 30′ followed by the preconditioningregion 32′ and primary stretching region 34′. Within the preconditioningregion 32′ (or optionally in the supply region 30′) a gripping memberset zone 31′ is provided in which the tracks diverge slightly to set thegripping members (for example, tenter clips) on the film. The film isoptionally heated within this zone. This initial TD stretch is typicallyno more than 5% of the final TD stretch and generally less than 2% ofthe final TD stretch and often less than 1% of the final TD stretch. Insome embodiments, the zone in which this initial stretch occurs isfollowed by a zone 33′ in which the tracks are substantially paralleland the film is heated or maintained at an elevated temperature.

Returning to FIG. 5, the film is stretched in the primary stretchingregion 34. Typically, within the primary stretching region 34 the filmis heated or maintained in a heated environment above the glasstransition of the polymer(s) of the film. For polyesters, thetemperature range is typically between 80° C. and 160° C. Examples ofsuitable heating elements include convective and radiative heatingelements, although other heating elements can also be used. In someembodiments, the heating elements used to heat the film can becontrolled individually or in groups to provide a variable amount ofheat. Such control can be maintained by a variety of processes includingvariability in the temperature of the heating elements or in thedirection or speed of air directed from the heating element to the film.The control of the heating elements can be used, if desired, to variablyheat regions of the film to improve or otherwise alter uniformity ofstretching across the film. For example, areas of the film that do notstretch as much as other areas under uniform heating can be heated moreto allow easier stretching.

Within the primary stretching region 34, the gripping members followgenerally diverging tracks to stretch the polymer film by a desiredamount. The tracks in the primary stretching region and in other regionsof the apparatus can be formed using a variety of structures andmaterials. Outside of the primary stretching region, the tracks aretypically substantially linear. The opposing linear tracks can beparallel or can be arranged to be converging or diverging. Within theprimary stretching region, the tracks are generally diverging and aregenerally curvilinear, as described below.

In all regions of the stretching apparatus, the tracks can be formedusing a series of linear or curvilinear segments that are optionallycoupled together. The tracks can be made using segments that allow twoor more (or even all) of the individual regions to be separated (forexample, for maintenance or construction). As an alternative or inparticular regions or groups of regions, the tracks can be formed as asingle continuous construction. The tracks can include a continuousconstruction spanning one or more adjacent regions of the stretcher. Thetracks can be any combination of continuous constructions and individualsegments.

In at least some embodiments, the tracks in the primary stretchingregion are coupled to, but separable from, the tracks of the precedingregions. The tracks 140, 141 in the succeeding post-conditioning orremoval regions are typically separated from the tracks of the primarystretching region, as illustrated, for example, in FIG. 5.

Although the tracks in the primary stretching region are curvilinear,linear track segments can be used in at least some embodiments. Thesesegments are aligned (by, for example, pivoting individual linearsegments about an axis) with respect to each other to produce a linearapproximation to a desired curvilinear track configuration. Generally,the shorter the linear segments are, the better the curvilinearapproximation can be made. In some embodiments, the positions of one ormore, and preferably all, of the linear segments are adjustable(pivotable about an axis) so that the shape of the tracks can beadjusted if desired. Adjustment can be manual or the adjustment can beperformed mechanically, preferably under control of a computer or otherdevice coupled to a driver. It will be understood, that curvilinearsegments can be used instead of or in addition to linear segments.

Continuous tracks can also be used through each of the regions. Inparticular, a continuous, curvilinear track can be used through theprimary stretching region. The continuous, curvilinear track typicallyincludes at least one continuous rail that defines the track along whichthe gripping members run. In one embodiment, the curvilinear trackincludes two pairs of inner and outer rails with tenter clips mounted ontop and bottom bearings rolling between the four rails.

In some embodiments, the continuous track is adjustable. One method ofmaking an adjustable continuous track includes the use of one or moretrack shape control units. These track shape control units are coupledto a portion of the continuous track, such as the continuous rail, andare configured to apply a force to the track as required to bend thetrack. FIG. 9 schematically illustrates one embodiment of such anarrangement with the track shape control units 65 coupled to the track64. Generally, the track shape control units have a range of forces thatthe track shape control unit can apply, although some embodiments may belimited to control units that are either on or off. The track shapecontrol units can typically apply a force toward the center of the filmor apply a force away from the center of the film or, preferably, both.The track shape control units can be coupled to a particular point onthe adjustable continuous track or the track shape control units can beconfigured so that the track can slide laterally along the control unitwhile still maintaining coupling between the track and control unit.This arrangement can facilitate a larger range of motion because itallows the track to more freely adjust as the control units areactivated. Generally, the track shape control units allow the track tomove through a range of shapes, for example, shapes 67 and 69 of FIG. 9.Typically, the track shape control unit and the track can move along aline (or other geometric shape) of motion. When more than one trackshape control unit is used, the track shape control units can have thesame or similar lines of motion and ranges of motion or the lines andranges of motions for the individual track shape control units can bedifferent.

One example of a suitable track shape control unit and track isillustrated in FIG. 16. The track in this embodiment includes four rails400 with tenter clips (not shown) mounted on bearings (not shown)rolling between the four rails. The track shape control unit includes abase 402 that is coupled to a driver (not shown), top and bottom innercontact members 404, and top and bottom outer contact members 406. Theinner and outer contact members 404, 406 are coupled to the base 402 sothat moving the base allows the contact members to apply a force toinner and outer surfaces of the rails, respectively. Preferably, theinner and outer contact members have a shape, when viewed from above orbelow, that provides only small areas of contact between the innercontact members 406 an the rails 400, as illustrated in FIG. 21 (whichonly shows the rails 400 and inner contact member 406). Examples of suchshapes include circular and ovoid, as well as diamond, hexagonal, orother similar shapes where contact between the inner contact members 406and the rails is made at the apex of these shapes. The outer contactmembers 404 can be similarly fashioned so that the portion of the outercontact member, when viewed from above or below, comes to a point tomake contact with the rails 400, as illustrated in FIG. 22 (which onlyshows the rails 400 and the portion of the outer contact member 404 thatmakes contact with the rails). Using such shapes allows the track shapecontrol unit to exert a force, if desired, to modify the track shapewhile allowing the track to slide laterally through the control unitrather than being fixed to the control unit. This configuration can alsoallow the track to adjust its instantaneous slope within the controlunit. For one or both of these reasons, the track can have a largerrange of shape adjustment. In other embodiments, there can be fewer ormore contact members or there may be only inner or only outer contactmembers.

Returning to FIG. 9, in some embodiments, one or more points 73 of thetrack are fixed. The fixed points can be anywhere along the trackincluding at or near the start (as illustrated in FIG. 9) or end of theprimary stretching region. The fixed points 73 can also be positioned atother points along the track as illustrated in FIG. 15.

As further illustrated in FIG. 15, the tracks can be configured toprovide zones 81, 83, 85 within the primary stretching region that havedifferent stretching characteristics or that can be described bydifferent mathematical equations. In some embodiments, the tracks have ashape that defines these different zones. In other embodiments, thetracks can be adjusted, using for example the track shape control unitsdiscussed above, to provide a variety of shapes 87, 89 beyond simple,monofunctional arrangements. This can be advantageous because it allowsdifferent portions of the primary stretching region to accomplishdesired functions. For example, an initial stretching zone may have aparticular shape (for example, a super-uniaxial shape with U>1 and F>1as described below) followed by one or more later zones with differentshapes (for example, a uniaxial shape). Optionally, intermediate zonescan be provided that transition from one shape to another. In someembodiments, the individual zones can be separated or defined by points73 of the track that are fixed.

In some embodiments, the track has a non-uniform cross-sectional shapealong the length of the track to facilitate bending and shaping of thetrack. For example, one or more rails used in the track can havedifferent cross-sectional shapes. As an example, in the four railconstruction described above each of the rails, or a subset of therails, has a varied cross-section along the length of the track. Thecross-section can be varied by, for example, altering either the heightor thickness of the track (or a component of the track such as one ormore continuous rails) or both. As an example, in one embodiment thethickness of the track or one or more rails in the track decreases orincreases along the length of the track in the machine direction. Thesevariations can be used to support a particular track shape or avariation in track shape adjustability. For example, as described abovethe track may have several different zones, each zone having a differenttrack shape. The cross-sectional variation of the track or component ofthe track can vary within each zone to achieve or facilitate aparticular rail shape and can vary between zones. As an example, a zonewith a relatively thick cross-sectional shape can be disposed betweentwo other zones to isolate or provide a transitional space between thetwo zones.

As an example of variation in track or rail cross-section, thearclength, s, can be used to represent a position along the track in thedesign of the thickness profile of a track or portion of a track, suchas a rail. The arclength, s, at the start of draw is defined as zero andat the other end of the draw is defined as L with correspondingthicknesses at the beginning and end of draw being designated as h(0)and h(L), respectively. The track or track component (e.g., rail) inthis particular embodiment has a taper over a portion of the beam fromL′ to L″ between s=0 and s=L such that the thickness h(L′) at positionL′ is greater than the thickness h(L″) at position L″. In this manner,either L′ or L″ may be at the higher arclength coordinate (i.e., L′>L″or L′<L″). One example of a useful thickness profile is a taper given bythe function for thickness, h(s), as a function of arclength s over therail from L′ to L″ is provided by the equation:h(s)=(h(L′)−h(L″))(1−(s−L′)/(L″−L′))^(α) +h(L″)where α is the positive rate of taper resulting in decreasing thicknessfrom L′ to L″. When L′ is less than L″ this results in a decreasingthickness with arclength. When L′ is greater than L″ this results in aincreasing thickness with arclength. The track can optionally beapportioned into sections, each with its own local L′, L″ and rate oftaper. The maximum thickness of the track or track component depends onthe amount of flexibility desired at that point on the track. Using beamtheory, it can be shown that in the case of a straight beam with ataper, a value for α of one third provides a beam that bendsparabolically in response to a load at one end. When the beam begins ina curved equilibrium configuration or is loaded by several controlpoints, other tapers may be more desirable. For transformation across avariety of other shapes, it may be useful to have both increasing anddecreasing thickness within a given track or track component, ornumerically calculated forms of the taper over any of these sections.The minimum thickness at any point along the track or track componentdepends on the amount of required strength of the track to support thedrawing forces. The maximum thickness can be a function of the level ofneeded flexibility. It is typically beneficial to maintain the level oftrack adjustment within the elastic range of the track or trackcomponent, e.g. to avoid the permanent yielding of the track or trackcomponent and loss of repeatable adjustment capability.

The paths defined by the opposing tracks affect the stretching of thefilm in the MD, TD, and ND directions. The stretching (or drawing)transformation can be described as a set of draw ratios: the machinedirection draw ratio (MDDR), the transverse direction draw ratio (TDDR),and the normal direction draw ratio (NDDR). When determined with respectto the film, the particular draw ratio is generally defined as the ratioof the current size (for example, length, width, or thickness) of thefilm in a desired direction (for example, TD, MD, or ND) and the initialsize (for example, length, width, or thickness) of the film in that samedirection. Although these draw ratios can be determined by observationof the polymer film as drawn, unless otherwise indicated reference toMDDR, TDDR, and NDDR refers to the draw ratio determined by a track usedto stretch the polymer film.

At any given point in the stretching process, TDDR corresponds to aratio of the current separation distance of the boundary trajectories,L, and the initial separation distance of the boundary trajectories, L₀,at the start of the stretch. In other words, TDDR=L/L₀. In someinstances (as in FIGS. 2 and 4), TDDR is represented by the symbol λ. Atany given point in the stretching process, MDDR is the cosine of thedivergence angle, θ, the positive included angle between MD and theinstantaneous tangent of the boundary trajectory, e.g. track or rail. Itfollows that cot(θ) is equal to the instantaneous slope (i.e., firstderivative) of the track at that point. Upon determination of TDDR andMDDR, NDDR=1/(TDDR*MDDR) provided that the density of the polymer filmis constant during the stretching process. If, however, the density ofthe film changes by a factor of ρ_(f), where ρ_(f)=ρ/ρ₀ with ρ being thedensity at the present point in the stretching process and ρ₀ being theinitial density at the start of the stretch, then NDDR=ρ_(f)/(TDDR*MDDR)as expected. A change in density of the material can occur for a varietyof reasons including, for example, due to a phase change, such ascrystallization or partial crystallization, caused by stretching orother processing conditions.

Perfect uniaxial drawing conditions, with an increase in dimension inthe transverse direction, result in TDDR, MDDR, and NDDR of λ,(λ)^(−1/2), and (λ)^(−1/2), respectively, as illustrated in FIG. 2(assuming constant density of the material). In other words, assuminguniform density during the draw, a uniaxially oriented film is one inwhich MDDR=(TDDR)^(−1/2) throughout the draw. A useful measure of theextent of uniaxial character, U, can be defined as:$U = \frac{\frac{1}{MDDR} - 1}{{TDDR}^{1/2} - 1}$For a perfect uniaxial draw, U is one throughout the draw. When U isless than one, the drawing condition is considered “subuniaxial”. When Uis greater than one, the drawing condition is considered“super-uniaxial”. In a conventional tenter, where the polymer film isstretched linearly along tracks 2, as illustrated in FIGS. 1 and 2, tostretch a region 4 of the film to a stretched region 6 and thedivergence angle is relatively small (e.g., about 30 or less), MDDR isapproximately 1 and U is approximately zero. If the film is biaxiallydrawn so that MDDR is greater than unity, U becomes negative. In someembodiments, U can have a value greater than one. States of U greaterthan unity represent various levels of over-relaxing. These over-relaxedstates produce an MD compression from the boundary edge. If the level ofMD compression is sufficient for the geometry and material stiffness,the film will buckle or wrinkle.

As expected, U can be corrected for changes in density to give U_(f)according to the following formula:$U_{f} = \frac{\frac{1}{MDDR} - 1}{( \frac{TDDR}{\rho_{f}} )^{1/2} - 1}$

Preferably, the film is drawn in plane (i.e., the boundary trajectoriesand tracks are coplanar) such as shown in FIG. 5, although non-coplanarstretching trajectories are also acceptable. The design of in-planeboundary trajectories is simplified because the in-plane constraintreduces the number of variables. The result for a perfect uniaxialorientation is a pair of mirror symmetric, in-plane, parabolictrajectories diverging away from the in-plane MD centerline. Theparabola may be portrayed by first defining TD as the “x” direction andMD as the “y” direction. The MD centerline between the opposing boundingparabolas may be taken as the y coordinate axis. The coordinate originmay be chosen to be the beginning of the primary stretching region andcorresponds to the initial centerpoint of the central trace between theparabolic trajectories. The left and right bounding parabolas are chosento start (y=0) at minus and plus x₀, respectively. The right boundingparabolic trajectory, for positive y values, that embodies thisembodiment of the invention is:x/x ₀=(¼) (y/x ₀)²+1The left bounding parabolic trajectory is obtained by multiplying theleft-hand side of the above equation by minus unity. In the discussionbelow, descriptions of and methods for determining the right boundedtrajectory are presented. A left bounded trajectory can then be obtainedby taking a mirror image of the right bounded trajectory over thecenterline of the film.

The coplanar parabolic trajectory can provide uniaxial orientation underideal conditions. However, other factors can affect the ability toachieve uniaxial orientation including, for example, non-uniformthickness of the polymer film, non-uniform heating of the polymer filmduring stretching, and the application of additional tension (forexample, machine direction tension) from, for example, down-web regionsof the apparatus. In addition, in many instances it is not necessary toachieve perfect uniaxial orientation. Instead, a minimum or threshold Uvalue or an average U value that is maintained throughout the draw orduring a particular portion of the draw can be defined. For example, anacceptable minimum/threshold or average U value can be 0.7, 0.75, 0.8,0.85, 0.9, or 0.95, as desired, or as needed for a particularapplication.

As an example of acceptable nearly uniaxial applications, the off-anglecharacteristics of reflective polarizers used in liquid crystallinedisplay applications is strongly impacted by the difference in the MDand ND indices of refraction when TD is the principal mono-axial drawdirection. An index difference in MD and ND of 0.08 is acceptable insome applications. A difference of 0.04 is acceptable in others. In morestringent applications, a difference of 0.02 or less is preferred. Forexample, the extent of uniaxial character of 0.85 is sufficient in manycases to provide an index of refraction difference between the MD and NDdirections in polyester systems containing polyethylene naphthalate(PEN) or copolymers of PEN of 0.02 or less at 633 nm for mono-axiallytransverse drawn films. For some polyester systems, such as polyethyleneterephthalate (PET), a lower U value of 0.80 or even 0.75 may beacceptable because of lower intrinsic differences in refractive indicesin non-substantially uniaxially drawn films.

For sub-uniaxial draws, the final extent of truly uniaxial character canbe used to estimate the level of refractive index matching between the y(MD) and z (ND) directions by the equationΔn _(yz) =Δn _(yz)(U=0)×(1−U)where Δn_(yz) is the difference between the refractive index in the MDdirection (i.e., y-direction) and the ND direction (i.e., z-direction)for a value U and Δn_(yz)(U=0) is that refractive index difference in afilm drawn identically except that MDDR is held at unity throughout thedraw. This relationship has been found to be reasonably predictive forpolyester systems (including PEN, PET, and copolymers of PEN or PET)used in a variety of optical films. In these polyester systems,Δn_(yz)(U=0) is typically about one-half or more the differenceΔn_(xy)(U=0) which is the refractive difference between the two in-planedirections MD (y-axis) and TD (x-axis). Typical values for Δn_(xy)(U=0)range up to about 0.26 at 633 nm. Typical values for Δn_(yz)(U=0) rangeup to 0.15 at 633 nm. For example, a 90/10 coPEN, i.e. a copolyestercomprising about 90% PEN-like repeat units and 10% PET-like repeatunits, has a typical value at high extension of about 0.14 at 633 nm.Films comprising this 90/10 coPEN with values of U of 0.75, 0.88 and0.97 as measured by actual film draw ratios with corresponding values ofΔn_(yz) of 0.02, 0.01 and 0.003 at 633 nm have been made according tothe methods of the present invention.

One set of acceptable parabolic trajectories that are nearly orsubstantially uniaxial character can be determined by the followingmethod. This described method determines the “right” boundary trajectorydirectly, and the “left” boundary trajectory is taken as a mirror image.First, a condition is set by defining an instantaneous functionalrelationship between TDDR measured between the opposing boundarytrajectories and MDDR defined as the cosine of the non-negativedivergence angle of those boundary trajectories, over a chosen range ofTDDR. Next, the geometry of the problem is defined as described in thediscussion of the parabolic trajectories. x₁ is defined as the initialhalf distance between the boundary trajectories and a ratio (x/x₁) isidentified as the instantaneous TDDR, where x is the current x positionof a point on the boundary trajectory. Next, the instantaneousfunctional relationship between the TDDR and MDDR is converted to arelationship between TDDR and the divergence angle. When a specificvalue of U is chosen, the equations above provide a specificrelationship between MDDR and TDDR which can then be used in thealgorithm to specify the broader class of boundary trajectories thatalso includes the parabolic trajectories as a limiting case when Uapproaches unity. Next, the boundary trajectory is constrained tosatisfy the following differential equation:d(x/x ₁)/d(y/x ₁)=tan(θ)where tan(θ) is the tangent of the divergence angle θ, and y is the ycoordinate of the current position of the opposing point on the rightboundary trajectory corresponding to the given x coordinate. Next, thedifferential equation may be solved, e.g. by integrating 1/tan(θ) alongthe history of TDDR from unity to the maximum desired value to obtainthe complete coordinate set {(x,y)} of the right boundary trajectory,either analytically or numerically.

As another example of acceptable trajectories, a class of in-planetrajectories can be described in which the parabolic trajectory is usedwith smaller or larger initial effective web TD length. If x₁ is half ofthe separation distance between the two opposing boundary trajectoriesat the inlet to the primary stretching region (i.e. the initial film TDdimension minus the selvages held by the grippers which is the initialhalf distance between opposing boundary trajectories), then this classof trajectories is described by the following equation:±(x)/(x ₁)=(¼)(x ₁ /x ₀)(y/x ₁)²⁺¹where x₁/x₀ is defined as a scaled inlet separation. The quantity x₀corresponds to half of the separation distance between two opposingtracks required if the equation above described a parabolic tracks thatprovided a perfectly uniaxial draw. The scaled inlet separation, x₁/x₀,is an indication of the deviation of the trajectory from the uniaxialcondition. In one embodiment, the distance between the two opposingtracks in the primary stretching zone is adjustable, as described above,allowing for the manipulation of the trajectory to provide values of Uand F different than unity. Other methods of forming these trajectoriescan also be used including, for example, manipulating the shape of thetrajectories using track shape control units or by selecting a fixedshape that has the desired trajectory.

For super-uniaxial draws, the severity of the wrinkling can bequantified using the concept of overfeed. The overfeed, F, can bedefined as the uniaxial MDDR (which equals (TDDR)^(−1/2)) divided by theactual MDDR. If the actual MDDR is less than the uniaxial MDDR, theoverfeed F is less than unity and the MDDR is under-relaxed resulting ina U less than unity. If F is greater than unity, the draw issuper-uniaxial and the MDDR is over-relaxed relative to the uniaxialcase. At least a portion of the extra slack can be accommodated as awrinkle because the compressive buckling threshold is typically low forthin, compliant films. When F is greater than unity, the overfeedcorresponds at least approximately to the ratio of the actual filmcontour length in the wrinkles along MD to the in-plane contour lengthor space.

Because of the relationship between TDDR and MDDR in the case ofconstant density, F can be written as:F=1/(MDDR×TDDR ^(1/2))Typically, F is taken as density independent for design purposes. Largevalues of F anytime during the process can cause large wrinkles that canfold over and stick to other parts of the film causing defects. In atleast some embodiments, the overfeed, F, remains at 2 or less during thedraw to avoid or reduce severe wrinkling or fold over. In someembodiments, the overfeed is 1.5 or less throughout the course of thedraw. For some films, a maximum value of F of 1.2 or even 1.1 is allowedthroughout the draw.

For at least some embodiments, particularly embodiments with U>1 throughthe entire draw, rearranging the definition of overfeed provides arelative bound on a minimum MDDR given a current TDDR:MDDR>1/(F _(max) ×TDDR ^(1/2))where F_(max) can be chosen at any preferred level greater than unity.For example, F can be selected to be 2, 1.5, 1.2, or 1.1, as describedabove.

When the over-feed is less than unity, there is effectively morein-plane space along MD than is desired for the truly uniaxial draw andthe MDDR may be under-relaxed and causing MD tension. The result can bea U value less than unity. Using the relationships between U, F, MDDRand TDDR there is a corresponding correlation between U and F whichvaries with TDDR. At a critical draw ratio of 2, a minimum U valuecorresponds to a minimum overfeed of about 0.9. For at least someboundary trajectories including boundary trajectories in which U>1 forthe entire draw, MDDR can be selected to remain below a certain levelduring a final portion of draw, e.g.MDDR<1/(F _(min) ×TDDR ^(1/2))where F_(min) is 0.9 or more for a final portion of draw after a drawratio of 2.

As an example, trajectories can be used in which MDDR<(TDDR)^(−1/2)(i.e., U>1) throughout the stretch, F_(max) is 2, and the film isstretched to a TDDR of at 4. If the trajectories are coplanar, then thefilm is stretched to a TDDR of at least 2.4 and often at least 5.3. IfF_(max) is 1.5, then the film is stretched to a TDDR of at least 6.8. Ifthe trajectories are coplanar, then the film is stretched to a TDDR ofat least 2.1 and often at least 4.7, If F_(max) is 1.2, then the film isstretched using coplanar trajectories to a TDDR of at least 1.8 andoften at least 4.0. For coplanar or non-coplanar boundary trajectories,if no limit is placed on F, then the film is stretched to a TDDR ofgreater than 4 and often of at least 6.8.

In another example, coplanar trajectories can be used in which(F_(min))*(MDDR)<(TDDR)^(−1/2) throughout the stretch, F_(max) is 2,F_(min) is 0.9, and the film is stretched to a TDDR of at least 4.6 andoften at least 6.8. If F_(max) is 1.5, then the film is stretched to aTDDR of at least 4.2 and often at least 6.1. If F_(max) is 1.2, then thefilm is stretched to a TDDR of at least 3.7 and often at least 5.4. Ifno limit is placed on F, then the film is stretched to a TDDR of atleast 8.4. A boundary trajectory can also be used in which(F_(min))*(MDDR)<(TDDR)^(−1/2) throughout the stretch, F_(max) is 1.5,F_(min) is 0.9, and the film is stretched to a TDDR of at least 6.8.

Other useful trajectories can be defined using F_(max). Usefultrajectories include coplanar trajectories where TDDR is at least 5, Uis at least 0.85 over a final portion of the stretch after achieving aTDDR of 2.5, and F_(max) is 2 during stretching. Useful trajectoriesalso include coplanar trajectories where TDDR is at least 6, U is atleast 0.7 over a final portion of the stretch after achieving a TDDR of2.5, and F_(max) is 2 during stretching.

Yet other useful coplanar trajectories include those in whichMDDR<TDDR^(−1/2)<(F_(max))*(MDDR) during a final portion of the draw inwhich TDDR is greater than a critical value TDDR′. The followingprovides minimum draw ratios that should be achieved for the trajectory.When TDDR′ is 2 or less, then for F_(max)=2, the minimum draw is 3.5;for F_(max)=1.5, the minimum draw is 3.2; and for F_(max)=2, the minimumdraw is 2.7. When TDDR′ is 4 or less, then for F_(max)=2, the minimumdraw is 5.8; for F_(max)=1.5, the minimum draw is 5.3; and forF_(max)=1.2, the minimum draw is 4.8. When TDDR′ is 5 or less, then forF_(max)=2, the minimum draw is 7; for F_(max)=1.5, the minimum draw is6.4; and for F_(max)=1.2, the minimum draw is 5.8.

In general, a variety of acceptable trajectories can be constructedusing curvilinear and linear tracks so that the overfeed remains below acritical maximum level throughout the drawing to prevent fold-overdefects while remaining above a critical minimum level to allow thedesired level of truly uniaxial character with its resulting properties.

A variety of sub-uniaxial and super-uniaxial trajectories may be formedusing the parabolic shape. FIG. 18 illustrates examples that demonstratea different levels minimum U after a critical TDDR and that demonstratea different maximum overfeeds up to a final desired TDDR. The curves arerepresented by coordinates x and y as scaled by x₁, half the initialseparation distance of the tracks. The scaled x coordinate, the quantity(x/x₁), is therefore equal to the TDDR. Curve 300 is the ideal case witha value of x₁/x₀ of 1.0. Curve 302 is the parabolic case with a value ofx₁/x₀ of 0.653 in which U remains greater than 0.70 above a draw ratioof 2.5. Curve 304 is the parabolic case with a value of x₁/x₀ of 0.822in which U remains above 0.85 after a draw ratio of 2.5. Curves 306,308, and 310 illustrate various levels of overfeed. The overfeed, TDDRand scaled inlet width are related byx ₁ /x ₀=(F ²(TDDR)−1)/(TDDR−1)It follows directly that the overfeed increases with increasing TDDR inthe parabolic trajectories described here. Curve 306 is the paraboliccase with a value of x₁/x₀ of 1.52 in which the overfeed remains below1.2 up to a final draw ratio of 6.5. Curve 308 is the parabolic casewith a value of x₁/x₀ of 2.477 in which the overfeed remains below 1.5up to a final draw ratio of 6.5. Curve 310 is the parabolic case with avalue of x₁/x₀ of 4.545 in which the overfeed remains below 2 up to afinal draw ratio of 6.5. The level of overfeed is a function of thefinal draw ratio in these cases. For example, using a value of x₁/x₀ ofonly 4.333 rather than 4.545 allows drawing to a final TDDR of 10 whilekeeping the overfeed under 2.

For the parabolic trajectories, a relationship allows the directcalculation of MDDR at any given TDDR for a fixed scaled inlet width:MDDR=(TDDR(x ₁ /x ₀)+(1−x ₁ /x ₀))^(−1/2)One observation is that the relationship between MDDR and TDDR is not anexplicit function of the y position. This allows the construction ofcomposite hybrid curves comprising sections of parabolic trajectoriesthat are vertically shifted in y/x₁. FIG. 19 illustrates one method. Aparabolic trajectory for the initial portion of the draw is chosen,curve 320 and a parabolic trajectory is chosen for the final portion,curve 322. The initial curve 320 is chosen to provide a super-uniaxialdraw with a maximum overfeed of 2.0 at a draw ratio of 4.5. Curve 320has a scaled inlet width of 4.857. The final curve 322 is chosen to be asub-uniaxial draw with a minimum U of 0.9 at the 4.5 draw ratio. Curve322 has a scaled inlet width of 0.868. The actual track or rail shapefollows curve 320 up to TDDR of 4.5 and then continues on curve 324which is a vertically shifted version of curve 322. In other words, atrajectory can have an initial stretching zone with tracks having afunctional form correspond to:±(x)/(x ₁)=(¼)(x ₁ /x ₀)(y/x ₁)²+1and then a later stretching zone with tracks having a functional formcorresponding to±(x)/(x ₂)=(¼)(x ₂ /x ₀)((y−A)/x ₂)²+1;where x₁ and x₂ are different and A corresponds to the vertical shiftthat permits coupling of the trajectories. Any number of parabolicsegments may be combined in this manner.

The parabolic trajectories, and their composite hybrids, can be used toguide the construction of related trajectories. One embodiment involvesthe use of linear segments to create trajectories. These linearapproximations can be constructed within the confines of parabolictrajectories (or composite hybrids) of maximum overfeed and minimumoverfeed (or minimum U) at a chosen TDDR′ larger than a critical drawratio, TDDR*. Values for TDDR* can be selected which relate to the onsetof strain-induced crystallinity with examples of values of 1.5, 2, and2.5 or may be related to elastic strain yielding with lower values of1.2 or even 1.1. The range of TDDR* generally falls between 1.05 and 3.Portions of the rail or track below TDDR* may not have any particularconstraints on minimum overfeed or U and may fall outside the confinesof the constraining parabolic trajectories. In FIG. 20, curve 340 ischosen to be the constraining parabolic trajectory of minimum overfeedat the chosen draw ratio, TDDR′, illustrated here at a value of 6.5. Forillustration, the minimum overfeed constraining parabolic trajectory hasbeen chosen as the ideal curve with a scaled inlet width of unity. Usingthe relationship between overfeed, TDDR and scaled inlet width, curve342 is identified as the constraining parabolic trajectory of maximumoverfeed where the maximum value of F is 2.0 at the TDDR value of 6.5.Curve 342 is now vertically shifted to form curve 344 so that the twoconstraining parabolic trajectories meet at the chosen TDDR′ of 6.5. Itshould be remarked that curves 342 and 344 are completely equivalentwith respect to drawing character. Curve 344 merely delays the stretchuntil a later spatial value of y/x₁ of 2.489. An approximation of linearor non-parabolic curvilinear segments will tend to lie between theseconstraining trajectories above TDDR*.

Unlike parabolic trajectories that possess increasing divergence angleswith increasing TDDR, linear trajectories have a fixed divergence angle.Thus the overfeed decreases with increasing TDDR along a linear segment.A simple linear approximation can be constructed by choosing a line witha divergence angle equal to the desired minimum overfeed at the chosenTDDR. The line segment may be extrapolated backwards in TDDR until theoverfeed equals the maximum allowed. A subsequent linear segment isstarted in similar fashion. The procedure is repeated as often asnecessary or desired. As the maximum overfeed decreases, the number ofsegments needed for the approximation increases. When the TDDR dropsbelow TDDR*, any number of methods may be used to complete the track orrail as long as the constraint on maximum overfeed is maintained. InFIG. 20, curve 346 is a linear approximation constrained by a maximumoverfeed of 2. Because of this large maximum overfeed, it comprises onlytwo linear sections. The final linear segment extends all the waybackwards from the chosen TDDR of 6.5 to a lower TDDR of 1.65. In thiscase, TDDR* is taken as 2. Without a constraint on U below a TDDR of 2,one method of finishing the track is to extrapolate a second linearsegment from TDDR at 1.65 back to TDDR of unity at the y/x₁ zero point.Note that this causes the second segment to cross the lower constrainingparabola, since the constraint is not effective below TDDR*.

In FIG. 20, curve 348 is the result of using a tighter value for themaximum overfeed of 1.5. Here the constraining parabolic trajectory ofmaximum overfeed is not shown. Three linear segments are required. Thefirst segment extends backwards from TDDR of 6.5 to TDDR of 2.9. Thesecond segment assumes a divergence angle equal to the constrainingparabolic trajectory of minimum overfeed at this TDDR value of 2.9 andextends backwards to a TDDR of 1.3. This second segment ends belowTDDR*. The final segment completes the track or rail shape for curve 348using a different method than that used for curve 346. Here the sameprocedure for the last segment is used as for the previous segments,resulting in a delay of the onset of stretching with a higher y/x₁value. A third method of completing the track is to set the overfeed tothe maximum at the initial TDDR of unity.

General, non-linear and non-parabolic trajectories fitting therequirements of the present invention can be constructed using theconstraining parabolic trajectories. The maximum overfeed constrainingparabolic trajectory is the curve of minimum slope, i.e. maximumdivergence angle, as a function of TDDR. The minimum overfeedconstraining parabolic trajectory is the curve of maximum slope, i.e.minimum divergence angle, as a function of TDDR. In general, curves canbe extrapolated backwards from the chosen TDDR′ using any function ofslope that lies between the constraining bounds. A simple method fordefining a function for the slope that lies between these constraints isto take a simple linear combination of known curves within the envelope.Curve 350 in FIG. 20 illustrates this simple method. In this example,350 is formed by a linear combination of the maximum overfeedconstraining parabolic trajectory, curve 344, and the linearapproximation to it, curve 346, with the linear weights of 0.7 and 0.3,respectively. In general, functions that are not simple linearcombinations can also be used.

The aforementioned method for describing the various non-parabolictrajectories of the present invention can be applied over differentsections of the track, e.g. the example of FIG. 20 for TDDR up to 6.5may be combined with another section for TDDR over 6.5 with differentrequirements and therefore different maximum and minimum constrainingtrajectories over that higher range of TDDR. In this case, the TDDR′ ofthe previous section of lower draw takes on the role of TDDR*. Ingeneral, TDDR′ may be chosen across the range of desired drawing.Various sections may be used to account for the various phenomenon ofdrawing, such as yielding, strain-induced crystallization, onset ofnecking or other draw non-uniformity, onset of strain-hardening or toaccount for the development of various properties within the film.Typical break points include those for TDDR*, the range of 3 to 7 forstrain-hardening in polyesters, and typical final draw values in therange of 4 to 10 or more.

The procedures for determining boundary trajectories for the presentinvention in the method of extrapolating backwards to lower TDDR from achosen TDDR′ may be used in an analogous method of extrapolating forwardto higher TDDR from a chosen TDDR″. Again, two constraining trajectoriesare formed, joined at the lowest chosen TDDR″. A convenient value forTDDR″ is the initial TDDR of unity. In this method, the constrainingtrajectory of minimum overfeed or U lies above the maximum overfeedcurve. FIG. 19 actually exhibits an example of this method in which thehybrid curve 324 lies between the minimum overfeed constraint, curve322, and the maximum overfeed constraint, curve 320.

Still another class of boundary trajectories can be defined and may, insome embodiments, be useful in suppressing residual wrinkles. Becausethe uniaxial condition in the absence of shear provides a principal MDstress of zero, it is anticipated, using finite strain analysis, thatthe principal MD stress will actually go into slight compression underthese conditions. Using finite strain analysis and a Neo-Hookean elasticsolid constitutive equation, it is discovered that a suitable criterionfor preventing compressive stresses may optionally be given by thefollowing equation:((TDDR)(MDDR))⁻⁴+((TDDR)(MDDR))²−(TDDR)⁻²−(MDDR)⁻²−sin²(θ)((TDDR)(MDDR))⁻²=0where MDDR is the cosine of the divergence angle. This optional methodof the present invention then specifies this class of boundarytrajectories.

As indicated above, the film may be drawn out-of-plane usingout-of-plane boundary trajectories, i.e. boundary trajectories that donot lie in a single Euclidean plane. There are innumerable, butnevertheless particular, boundary trajectories meeting relationalrequirements of this preferred embodiment of the present invention, sothat a substantially uniaxial draw history may be maintained usingout-of-plane boundary trajectories. The boundaries may be symmetrical,forming mirror images through a central plane, e.g. a plane comprisingthe initial center point between the boundary trajectories, the initialdirection of film travel and the initial normal to the unstretched filmsurface. In this embodiment, the film may be drawn between the boundarytrajectories along a cylindrical space manifold formed by the set ofline segments of shortest distance between the two opposing boundarytrajectories as one travels along these boundary trajectories at equalrates of speed from similar initial positions, i.e., colinear with eachother and the initial center point. The trace of this ideal manifold onthe central plane thus traces out the path of the film center for anideal draw. The ratio of the distance along this manifold from theboundary trajectory to this central trace on the central plane to theoriginal distance from the start of the boundary trajectory to theinitial center point is the instantaneous nominal TDDR across the filmspanning the boundary trajectories, i.e. the ratios of thehalf-distances between the current opposing points on the boundarytrajectories and the half-distances between the initial positions of theopposing points on the boundary trajectories. As two opposing pointsmove at constant and identical speeds along the opposing boundarytrajectories, the corresponding center point on the central tracechanges speed as measured along the arc of the central trace, i.e. thecurvilinear MD. In particular, the central trace changes in proportionwith the projection of the unit tangent of the boundary trajectory onthe unit tangent of the central trace.

The classes of trajectories described above are illustrative and shouldnot be construed as limiting. A host of trajectory classes areconsidered to lie within the scope of the present invention. Asindicated above, the primary stretching region can contain two or moredifferent zones with different stretching conditions. For example, onetrajectory from a first class of trajectories can be selected for aninitial stretching zone and another trajectory from the same first classof trajectories or from a different class of trajectories can beselected for each of the subsequent stretching zones.

The present invention encompasses all nearly uniaxial boundarytrajectories comprising a minimum value of U of about 0.7, morepreferably approximately 0.75, still more preferably about 0.8 and evenmore preferably about 0.85. The minimum U constraint may be applied overa final portion of the draw defined by a critical TDDR preferably ofabout 2.5, still more preferably about 2.0 and more preferably about1.5. In some embodiments, the critical TDDR can be 4 or 5. Above acritical TDDR, certain materials, e.g. certain monolithic and multilayerfilms comprising orientable and birefringent polyesters, may begin tolose their elasticity or capability of snap back because of thedevelopment of structure such as strain-induced crystallinity. Thecritical TDDR may coincide with a variety of material and process (e.g.temperature and strain rate) specific events such as the critical TDDRfor the onset of strain-induced crystallization. The minimum value of Uabove such a critical TDDR can relate to an amount of non-uniaxialcharacter set into the final film.

A variety of boundary trajectories are available when U is subuniaxialat the end of the stretching period. In particular, useful boundarytrajectories include coplanar trajectories where TDDR is at least 5, Uis at least 0.7 over a final portion of the stretch after achieving aTDDR of 2.5, and U is less than 1 at the end of the stretch. Otheruseful trajectories include coplanar and non-coplanar trajectories whereTDDR is at least 7, U is at least 0.7 over a final portion of thestretch after achieving a TDDR of 2.5, and U is less than 1 at the endof the stretch. Useful trajectories also include coplanar andnon-coplanar trajectories where TDDR is at least 6.5, U is at least 0.8over a final portion of the stretch after achieving a TDDR of 2.5, and Uis less than 1 at the end of the stretch. Useful trajectories includecoplanar and non-coplanar trajectories where TDDR is at least 6, U is atleast 0.9 over a final portion of the stretch after achieving a TDDR of2.5, and U is less than 1 at the end of the stretch.

Useful trajectories also include coplanar and non-coplanar trajectorieswhere TDDR is at least 7 and U is at least 0.85 over a final portion ofthe stretch after achieving a TDDR of 2.5.

In some embodiments, a small level of MD tension is introduced into thestretching process to suppress wrinkling. Generally, although notnecessarily, the amount of such MD tension increases with decreasing U.

In some embodiments, it is useful to increase the tension as the drawproceeds. For example, a smaller value of U earlier in the draw may tendto set more non-uniaxial character into the final film. Thus it may beadvantageous to combine the attributes of various trajectory classesinto composite trajectories. For example, a uniaxial parabolictrajectory may be preferred in the earlier portions of the draw, whilethe later portions of the draw may converge on a different trajectory.In another arrangement, U may be taken as a non-increasing function withTDDR. In still another arrangement, the overfeed, F, may be anon-increasing function with TDDR after a critical draw ratio of, forexample, 1.5, 2, or 2.5.

The uniaxial parabolic trajectory assumes a uniform spatial drawing ofthe film. Good spatial uniformity of the film can be achieved with manypolymer systems with careful control of the crossweb and downweb caliper(thickness) distribution of the initial, undrawn film or web, coupledwith the careful control of the temperature distribution at the start ofand during the draw. For example, a uniform temperature distributionacross the film initially and during draw on a film of initially uniformcaliper should suffice in most cases. Many polymer systems areparticularly sensitive to non-uniformities and will draw in anon-uniform fashion if caliper and temperature uniformity areinadequate. For example, polypropylenes tend to “line draw” undermono-axial drawing. Certain polyesters, notably polyethylenenaphthalate, are also very sensitive.

Non-uniform film stretching can occur for a variety of reasonsincluding, for example, non-uniform film thickness or other properties,non-uniform heating, etc. In many of these instances, portions of thefilm near the gripping members draws faster than that in the center.This creates an MD tension in the film that can limit ability to achievea final uniform MDDR. One compensation for this problem is to modify theparabolic or other uniaxial trajectory to present a lower MDDR. In otherwords, MDDR<(TDDR)^(−1/2) for a portion or all of the draw.

In one embodiment, a modified parabolic or other uniaxial trajectory isselected in which MDDR<(TDDR)^(−1/2), corresponding to a largerdivergence angle, for all of the draw. In at least some instances, thiscondition can be relaxed because a U value of less than unity isacceptable for the application. In such instances, a modified parabolicor other uniaxial trajectory is selected in which(0.9)MDDR<(TDDR)^(−1/2).

In another embodiment, a modified parabolic or other uniaxial trajectoryis selected in which MDDR<(TDDR)^(−1/2) for an initial stretching zonein which the TDDR is increase by at least 0.5 or 1. A differenttrajectory is then maintained for the remainder of the draw. Forexample, a later stretching zone (within the stretching region 34) wouldhave a parabolic or other uniaxial trajectory in which MDDR is equal toor approximately equal to (within ±5% and, preferably, within ±3%)(TDDR)^(−1/2). As an example, the initial stretching zone can accomplisha TDDR level up to a desired value. This desired value is typically nomore than 4 or 5. The later stretching zone can then increase the TDDRfrom the desired value of the initial stretching zone (or from a highervalue if there are intervening stretching zones). Generally, the laterstretching zone is selected to increase the TDDR value by 0.5 or 1 ormore.

Again, in at least some instances, the MDDR and TDDR relationship can berelaxed because a U value of less than unity is acceptable for theapplication. In such instances, the modified parabolic or other uniaxialtrajectory of the initial stretching zone is selected in which(0.9)MDDR<(TDDR)^(−1/2).

Returning to FIG. 5, the apparatus typically includes apost-conditioning region 36. For example, the film may be set in zone 48and quenched in zone 50. In some embodiments, quenching is performedoutside the stretching apparatus. Typically, the film is set when atleast one component of the film, e.g. one layer type in a multilayerfilm, reaches a temperature below the glass transition. The film isquenched when all components reach a temperature level below their glasstransitions. In the embodiment illustrated in FIG. 5, a takeaway systemis used to remove the film from the primary stretching region 34. In theillustrated embodiment, this takeaway system is independent of (i.e.,not directly connected to) the tracks upon which the film was conveyedthrough the primary stretching region. The takeaway system can use anyfilm conveyance structures such as tracks 140, 141 with gripping memberssuch as, for example, opposing sets of belts or tenter clips.

In some embodiments as illustrated in FIG. 10, TD shrinkage control canbe accomplished using tracks 140′, 141′ which are angled (as compared toparallel tracks 140, 141 that could be used in other embodiments of asuitable take-away system). For example, the tracks of the take-awaysystem can be positioned to follow a slowly converging path (making anangle θ of no more than about 5°) through at least a portion of thepost-conditioning region to allow for TD shrinkage of the film withcooling. The tracks in this configuration allow the control of TDshrinkage to increase uniformity in the shrinkage. In other embodiments,the two opposing tracks can be diverging typically at an angle of nomore than about 30 although wider angles can be used in someembodiments. This can be useful to increase the MD tension of the filmin the primary stretching region to, for example, reduce propertynon-uniformity such as the variation of principal axes of refractiveindex across the film.

In some embodiments, the position of the take-away system can beadjustable to vary the position along the stretching apparatus at whichthe take-away system grips the film, as illustrated in FIG. 11. Thisadjustability provides one way to control the amount of stretching towhich the film is subjected. Film received by tracks 140′, 141′ of atake-away system earlier in the draw (shown by dotted lines in FIG. 11)will generally have a smaller TDDR than would film received by a tracks140, 141 of a take-away system positioned later in the draw (shown insolid lines in FIG. 11). The take-away system can also, optionally, beconfigured to allow adjustment in the distance between the opposingtracks of the take-away system. In addition, the take-away system canalso, optionally, be configured to allow adjustment in the length of thetake-away system.

Another example of a possible take-away system includes at least twodifferent regions with separated tracks 140, 141, 142, 143. Theseregions can be formed using two separate sets 140, 141 and 142, 143 ofopposing tracks as illustrated in FIG. 12. In one embodiment,illustrated in FIG. 12, the first region can include tracks 140, 141that are disposed at a convergence angle to provide TD shrinkage controland the tracks 142, 143 in the second regions can be parallel. In otherembodiments, the opposing tracks of the two different regions can be setat two different convergence angles to provide TD shrinkage control, asdescribed above, or the first region can have parallel tracks and thesecond region have tracks disposed at a convergence angle to provide TDshrinkage control. Alternatively or additionally, the two differenttracks can be set at two different takeaway speeds to decouple theprimary stretching region from a takeaway region that applies tension toremove wrinkles.

In one embodiment the take-away system illustrated in FIG. 12, thetracks 142′, 143′ are nested within the opposing tracks 140, 141 priorto receiving the film. When the film is initially received by theopposing tracks 140, 141, the tracks 142, 143 move to the positionillustrated in FIG. 12. In other embodiments, the opposing tracks 140,141, 142, 143 are positioned as illustrated in FIG. 12 (i.e., notnested) in the absence of any film.

Another example of a take-away system is illustrated in FIG. 13. In thisexample, the tracks 140, 141 of the take-away system are angled withrespect to the centerline of the film as the film is conveyed throughthe tracks 64 of the primary stretching region. The angle of the twoopposing conveyance mechanisms can be the same, for example, an angle βor the angle can be different and can be described as β+ε for one trackand β−ε for the other track. Typically, β is at least 1° and can be anangle of 5°, 10°, or 20° degrees or more. The angle ε would correspondto the converging or diverging angle described above to provide TDshrinkage control, etc. In some embodiments, the tracks 64 in theprimary stretching zone can also be disposed at an angle φ and thetracks 140, 141 are angled at φ+β+ε and φ+β−ε as illustrated in FIG. 13.An angled take-away system, primary stretching zone, or both can beuseful to provide films where the principal axis or axes of an propertyof the film, such as the refractive index axes or tear axis, is angledwith respect to the film. In some embodiments, the angle that thetake-away system makes with respect to the primary stretching zone isadjustable manually or mechanically using a computer-controlled driveror other control mechanism or both.

In some embodiments using an angled take-away system, the two opposingtracks are positioned to receive film having the same or substantiallysimilar TDDR (where the dotted line indicates film at the same TDDR), asillustrated in FIG. 13. In other embodiments, the two opposing tracks140, 141 are positioned to receive the film so that the TDDR isdifferent for the two opposing tracks (the dotted line of FIG. 14indicates film at the same TDDR), as illustrated in FIG. 14. This latterconfiguration can provide a film with properties that change over the TDdimension of the film.

Typically, the portions of the film that were held by the grippingmembers through the primary stretching region are removed. To maintain asubstantially uniaxial draw throughout substantially all of the drawhistory (as shown in FIG. 5), at the end of the transverse stretch, therapidly diverging edge portions 56 are preferably severed from thestretched film 48 at a slitting point 58. A cut can be made at 58 andflash or unusable portions 56 can be discarded.

Release of the selvages from a continuous gripping mechanism can be donecontinuously; however, release from discrete gripping mechanisms, suchas tenter clips, should preferably be done so that all the materialunder any given clip is released at once. This discrete releasemechanism may cause larger upsets in stress that may be felt by thedrawing web upstream. In order to assist the action of the isolatingtakeaway device, it is preferred to use a continuous selvage separationmechanism in the device, e.g. the “hot” slitting of the selvage from thecentral portion of a heated, drawn film.

The slitting location is preferably located near enough to the“gripline”, e.g. the isolating takeaway point of first effective contactby the gripping members of the take-away system, to minimize or reducestress upsets upstream of that point. If the film is slit before thefilm is gripped by the take-away system, instable takeaway can result,for example, by film “snapback” along TD. The film is thus preferablyslit at or downstream of the gripline. Slitting is a fracture processand, as such, typically has a small but natural variation in spatiallocation. Thus it may be preferred to slit slightly downstream of thegripline to prevent any temporal variations in slitting from occurringupstream of the gripline. If the film is slit substantially downstreamfrom the gripline, the film between the takeaway and boundary trajectorywill continue to stretch along TD. Since only this portion of the filmis now drawing, it now draws at an amplified draw ratio relative to theboundary trajectory, creating further stress upsets that could propagateupstream, for example, undesirable levels of machine direction tensionpropagating upstream.

The slitting is preferably mobile and re-positionable so that it canvary with the changes in takeaway positions needed to accommodatevariable final transverse draw direction ratio or adjustment of theposition of the take-away system. An advantage of this type of slittingsystem is that the draw ratio can be adjusted while maintaining the drawprofile simply by moving the take-away slitting point 58.

A variety of slitting techniques can be used including a heat razor, ahot wire, a laser, a focused beam of intense IR radiation or a focusedjet of heated air. In the case of the heated jet of air, the air may besufficiently hotter in the jet to blow a hole in the film, e.g. by heatsoftening, melting and controlled fracture under the jet. Alternatively,the heated jet may merely soften a focused section of the filmsufficiently to localize further drawing imposed by the still divergingboundary trajectories, thus causing eventual fracture downstream alongthis heated line through the action of continued film extension. Thefocused jet approach may be preferred in some cases, especially when theexhaust air can be actively removed, e.g. by a vacuum exhaust, in acontrolled fashion to prevent stray temperature currents from upsettingthe uniformity of the drawing process. For example, a concentric exhaustring around the jet nozzle can be used. Alternatively, an exhaustunderneath the jet, e.g. on the other side of the film, can be used. Theexhaust may be further offset or supplemented downstream to furtherreduce stray flows upstream into the drawing zone.

Another attribute of the take-away system is a method of speed and or MDtension control so that the film can be removed in a manner compatiblewith the output speed. This take-away system could also be used to pullout any residual wrinkles in the film. The wrinkles could be initiallypulled out during start up by a temporary increase in the takeaway speedabove the output speed of the final, released portion of the drawn film,or the wrinkles could be pulled out by a constant speed above the outputfilm MD speed during continuous operation, e.g. in the case of asuper-uniaxial draw in the final portion of draw. The speed of thetakeaway can also be set above the MD velocity of the film along theboundary trajectories at the gripline. This can be used to alter theproperties of the film. This over-speed of the takeaway can also reducethe final value of U and is thereby limited by this consideration in thecontext of the final end use of the film.

The process also includes a removal portion in region 38. Optionally aroller 65 may be used to advance the film, but this may be eliminated.Preferably the roller 65 is not used as it would contact the stretchedfilm 52 with the attendant potential to damage the stretched film.Another cut 60 may be made and unused portion 61 may be discarded. Filmleaving the take-away system is typically wound on rolls for later use.Alternatively, direct converting may take place after take away.

The principles of MD and TD shrinkage control described above can alsobe applied to other stretching apparatuses including the conventionaltenter configuration illustrated in FIG. 1. FIG. 17 illustrated anembodiment in which the tracks 64 from a primary stretching region (suchas the linear diverging tracks illustrated in FIG. 1) continue into orthrough a portion of a post-conditioning region. The film is thenoptionally captured by an isolated takeaway system 140, 141, if desired.The continuation of the tracks 64 can be used to cool the film and allowfor shrinkage of the film. In some embodiments, the continued tracks 164follow a slowly converging path (making an angle θ of no more than about5°) through at least a portion of the post-conditioning region to allowfor TD shrinkage of the film with cooling. The tracks in thisconfiguration allow the control of TD shrinkage to increase uniformityin the shrinkage. In some embodiments, the tracks 264 follow a moreaggressively converging path (making an angle φ of at least 15°, andtypically in the range of 20° and 30°) through at least a portion of thepost-conditioning region to provide MD shrinkage control of the filmwith cooling. In some embodiments as illustrated in FIG. 17, thepost-conditioning region includes both slowly converging tracks 164 andmore aggressively converging tracks 264. In other embodiments, only oneset of tracks 164 and tracks 264 is used.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the instant specification.

1. An apparatus for processing a film, the apparatus comprising: aconveyor configured and arranged to convey the film along a machinedirection within the apparatus, the conveyor comprising gripping membersthat are configured and arranged to hold opposing edge portions of thefilm, and a portion of the conveyor is configured and arranged toprovide diverging paths along which the gripping members move to stretchthe film; and an isolated take-away system that receives the film fromthe conveyor after stretching the film, the take-away system comprisingopposing tracks and gripping members configured and arranged to graspopposing take-away regions of the film after a desired amount ofstretching and convey the opposing take-away regions of the film alongthe opposing tracks, wherein the opposing tracks define a region inwhich at least a portion of at least one of the opposing tracks isangled with respect to the machine direction.
 2. The apparatus of claim1, wherein the opposing tracks are angled with respect to each other. 3.The apparatus of claim 2, wherein the opposing tracks are angled towardeach other.
 4. The apparatus of claim 2, wherein the opposing tracks areangled away from each other.
 5. The apparatus of claim 2, wherein theopposing tracks are angled with respect to each other along an entirelength of the opposing tracks.
 6. The apparatus of claim 2, wherein thetake-away system is disposed at an angle relative to the machinedirection.
 7. The apparatus of claim 1, wherein the opposing tracks aredisposed at an angle of at least 1° with respect to the machinedirection.
 8. The apparatus of claim 7, wherein the opposing tracks aredisposed at different angles with respect to the machine direction. 9.The apparatus of claim 7, wherein the opposing tracks of the take-awaysystem are configured and arranged to grasp the opposing take-awayregions of the film at a position in which the opposing take-awayregions of the film have a same transverse directional draw ratio. 10.The apparatus of claim 7, wherein the opposing tracks of the take-awaysystem are configured and arranged to grasp the opposing take-awayregions of the film at a position in which the opposing take-awayregions of the film have different transverse directional draw ratios.11. The apparatus of claim 7, wherein the diverging paths of theconveyor are configured and arranged to stretch the film so that acenterline of the film is angled with respect to a machine direction ofthe conveyor prior to stretching the film.
 12. An apparatus forprocessing a film, the apparatus comprising: a conveyor comprisinggripping members that hold opposing edge portions of the film andconvey, under influence of a drive member, the film along a machinedirection within the apparatus, wherein a portion of the conveyor isconfigured and arranged to provide diverging paths along which thegripping members move to stretch the film; and an isolated take-awaysystem that receives the film from the conveyor after stretching thefilm, the take-away system comprising a first set of opposing tracks, asecond set of opposing tracks, a plurality of first gripping membersconfigured and arranged to grasp opposing first take-away regions of thefilm after a desired amount of stretching and convey the film along thefirst set opposing tracks, and a plurality of second gripping membersconfigured and arranged to grasp opposing second take-away regions ofthe film and convey the film along the second set of opposing tracks,wherein the second take-away regions are nearer a center of the filmthan the first take-away regions.
 13. The apparatus of claim 12, whereinthe second gripping members are configured and arranged to move from afirst position disposed at least partially between the first grippingmembers and a second position in which the second gripping members arenot partially disposed between the first gripping members.
 14. Theapparatus of claim 12, wherein the first gripping members are angledwith respect to each other.
 15. An apparatus for processing a film, theapparatus comprising: a conveyor comprising gripping members that holdopposing edge portions of the film and convey, under influence of adrive member, the film along a machine direction within the apparatus,wherein a portion of the conveyor is configured and arranged to providediverging paths along which the gripping members move to stretch thefilm; and an isolated take-away system that receives the film from theconveyor after stretching the film, the take-away system comprisingopposing tracks and a plurality of gripping members configured andarranged to grasp opposing take-away regions of the film after a desiredamount of stretching and convey the opposing take-away regions of thefilm along the opposing tracks, wherein the apparatus is configured andarranged to allow selection of a final transverse direction draw ratioof the film by changing a position of the isolated take-away system withrespect to a position of the conveyor.
 16. An apparatus for processing afilm, the apparatus comprising: a conveyor configured and arranged toconvey the film along a machine direction, the conveyor comprisinggripping members that are configured and arranged to hold opposing edgeportions of the film; a stretching region in which the gripping membersare configured and arranged to travel along diverging paths to stretchthe film; and a post-conditioning region disposed after the stretchingregion and comprising at least one zone in which the gripping membersare configured and arranged to travel along converging paths.
 17. Theapparatus of claim 16, wherein the post-conditioning region comprises atleast a first zone and a second zone, wherein the gripping members areconfigured and arranged to travel along paths converging at a firstangle in the first zone and along paths converging at a second angle inthe second zone, wherein the first and second angles are substantiallydifferent.
 18. The apparatus of claim 17, wherein the converging pathsin at least one zone of the post-conditioning region converge at anangle of no more than 3°.
 19. The apparatus of claim 17, wherein theconverging paths in at least one zone of the post-conditioning regionconverge at an angle of at least 15°.
 20. The apparatus of claim 17,wherein the converging paths in at least one zone of thepost-conditioning region converge at an angle of in the range of 20° to30°.